A fuel cell system is a system in which a fuel gas and an oxidizing agent gas are supplied to a fuel cell via corresponding gas feed passages, and electricity is generated by using an electrochemical reaction of these gases in a fuel cell body.
Water is generated in the fuel cell body by the electrochemical reaction, and this water is discharged from the fuel cell body in a condition that the water is contained in a fuel-off gas (a fuel gas discharged from the fuel cell body) and an oxidizing-agent-off gas (an oxidizing agent gas discharged from the fuel cell body).
Therefore, if the ambient temperature of the system falls to the freezing point or below in a stop state of the fuel cell system, the moisture in the gases remaining in the valves, the piping, etc., arranged in the gas passages of the system, may condense and the component parts of the system may be frozen. In such a case, there is a possibility that starting the fuel cell system is impossible even if an operation to start the fuel cell system is performed thereafter. Or, there is a possibility that, even if the fuel cell system can be started, it is difficult to perform the normal operation of the fuel cell system. In particular, the freezing of a gas supplying device, such as a pump, would make the supplying of fuel gas or oxidizing agent gas impossible, and considerable time would be needed to put the entire system into operation.
In a case in which a Roots type pump is used as a fuel feed pump of a fuel cell system, if the freezing of the pump arises in a condition that the moisture is contained between a clearance between the rotor and the opposing casing inside surface, the problem occurs that the rotor adheres to the casing and it is impossible to start operation of the pump promptly at a time of restarting the pump.
To avoid the problem, there has been proposed a Roots type pump in which a funnel-like accumulation part to accumulate the moisture is provided on the bottom of the casing and the water within the pump is discharged via the accumulation part. This pump is aimed at reducing the amount of residual water in the pump and preventing freezing of the condensed water in the pump at a time of stopping the pump. See Japanese Laid-Open Patent Application No. 2005-180421.
Moreover, there has been proposed a Roots type pump in which the forward/reverse rotation of the rotor is repeated, prior to a restart of the pump in low temperature environment, to separate or remove the frozen water between the rotor and the opposing casing inside surface, so that the pump is restarted thereafter. See Japanese Laid-Open Patent Application No. 2005-155409.
Patent document 1: Japanese Laid-Open Patent Application No. 2005-180421
Patent document 2: Japanese Laid-Open Patent Application No. 2005-155409